ore than two trillion wild and farmed fish are killed annually to feed the global population, yet their deaths often occur in silence, invisible to most of us. However, biology is clear on one point: fish are capable of suffering.
Take rainbow trout—a commonly farmed species worldwide—as an example. When killed by exposure to air, these fish don’t simply die. They endure a prolonged and intense period of distress.
A recent study published in Scientific Reports sheds new light on this suffering and offers a way forward.
Measuring Pain in Fish with a New Tool
Unlike greenhouse gases or human health, animal suffering lacks a clear metric. To fill that gap, researchers developed the Welfare Footprint Framework (WFF)—a system that quantifies pain in terms of minutes, helping compare suffering across species and circumstances.
The research team applied this tool to trout slaughter, a process where fish are often left to suffocate in open air.
When removed from water, a fish’s physiology spirals downward. Gills collapse, panic sets in, blood chemistry destabilizes, and the body begins shutting down—often over the course of 25 excruciating minutes.
Up to 25 Minutes of Measurable Pain
Researchers broke the trout’s suffering into four phases: from initial distress to the eventual shutoff of brain function.
Using neurological data and behavioral cues, they concluded that a typical trout experiences around 10 minutes of pain severe enough to be classified as harmful, disabling, or agonizing.
In extreme cases, the suffering can last more than 20 minutes. Per kilogram of fish, this equates to about 24 minutes of pain—roughly 11 minutes per pound.
To assess unconsciousness, scientists examined reflexes and brain activity. They also evaluated physiological signs like CO₂ buildup, acidosis, and muscle fatigue. Each stage of pain had defined thresholds—ranging from mild discomfort to total collapse of normal function.
Common Slaughter Methods Still Cause Suffering
Air asphyxiation remains legal and widespread. Alternatives like ice baths or ice slurries may sound humane but are often worse for cold-water fish like trout. The cold slows their metabolism, delaying loss of consciousness and prolonging pain.
Ice can also cause thermal shock, tissue injury, and psychological stress—extending suffering rather than easing it.
Pain often begins before the actual killing. Crowding, transport, and rough handling lead to injuries and stress that go largely unregulated but contribute heavily to overall suffering.
Stunning: A Path to Better Welfare
Two main stunning methods were studied: electrical and percussive. Properly applied electrical stunning could prevent 60 to 1,200 minutes of suffering per dollar invested, making it one of the most efficient ways to improve fish welfare.
Still, practical challenges remain. In real-world settings, incorrect voltage, poor equipment, or misplacement of electrodes often lead to failure, leaving fish conscious through slaughter.
Percussive stunning—delivering a precise blow to the head—works well in controlled conditions but is hard to scale. Different fish sizes and worker fatigue make consistency difficult. One mistake can mean the fish remains alert as it bleeds to death.
A New Framework for Understanding Animal Pain
The Welfare Footprint Framework offers more than just a score. It works with probabilities, acknowledging uncertainty in pain perception. For instance, if there’s a 40% chance pain is disabling and 40% it’s excruciating, both are factored in.
This makes the system more adaptable and realistic. Pain responses differ across individuals, even under the same conditions.
“The WFF gives us a scientifically grounded, transparent way to prioritize animal welfare improvements,” said Dr. Wladimir Alonso of the Welfare Footprint Institute. “It’s about maximizing impact.”
The model mirrors tools used in health and environmental policy—turning suffering into measurable, actionable units.
Why This Matters for Policy and the Public
Although slaughter represents a brief period in a fish’s life, it can contain the most intense suffering. And unlike long-term reforms in farming, slaughter improvements can be rapidly implemented—with potentially billions of animals affected.
The authors call for investment in reliable stunning methods and better training for workers. Policymakers can use this data to update outdated practices and implement minimum welfare standards.
Consumers also gain a new lens through which to view the ethics of their food choices.
Beyond Trout: A Global Concern
Though the study focused on rainbow trout, the mechanisms of pain—like oxygen loss, metabolic breakdown, and stress—apply across many fish species.
Salmon, catfish, tilapia, seabass—all may endure similar suffering under current methods. But species-specific research is still needed to adapt the framework more widely.
Some fish can tolerate low oxygen better than others. Some may respond more severely to cold. Future work will need to consider these variations.
A Shift in How We Treat Aquatic Life
Fish sentience was long ignored or denied. But today’s science clearly shows they are capable of pain—and the scale of their suffering is massive.
With trillions of fish dying each year, even small welfare improvements could make an enormous difference.
The Welfare Footprint Framework doesn’t just calculate pain—it creates a shared language to recognize and reduce it. For policymakers, producers, and consumers alike, it’s a tool to make more compassionate, informed decisions.
And for the fish, it’s a long-overdue acknowledgment: their suffering is real, measurable, and worth addressing.